Donetsk , Ukraine -LRB- CNN -RRB- -- A prominent leader of pro-Russian rebels in eastern Ukraine resigned his post Thursday as fighting flared there , with militants reportedly downing a Ukrainian military plane .

Alexander Borodai , prime minister of the self-proclaimed Donetsk People 's Republic , told reporters he was stepping down and handing over power to Alexander Zakharchenko , a little-known militia commander .

As Ukrainian forces rapidly advance toward the key rebel-controlled city and Russia builds up forces along the border , the sudden resignation leaves a number of questions about the volatile region 's future unanswered : What does it mean ? Are militants disbanding and vanishing into the night ? Is there a brutal battle ahead ?

Borodai , a Russian citizen who rose to power as pro-Russian rebels took portions of eastern Ukraine earlier this year , was a prominent public face for the separatists after Malaysia Airlines Flight 17 crashed last month .

He gave little reason for his resignation Thursday , other than saying he considered the job of founding the republic to be finished and that it was time for a Ukrainian to take the reins .

The announcement came after days of assault on rebel positions near Donetsk by Ukrainian forces . Shelling in the Donetsk area , which could be heard periodically in and around the city , killed four civilians and injured 18 others on Thursday , local officials said .

About 25 miles to the northeast , rebels shot down a Ukrainian MiG-29 near the town of Yenakievo on Thursday evening , Ukrainian military spokesman Vladislav Seleznyov said .

Initial information indicates that a missile from a Russian-made surface-to-air antiaircraft system , called Buk or SA-11 , shot the plane , Seleznyov said . Information about the fate of the pilot was n't immediately available .

That 's the same type of missile system that U.S. and Ukrainian officials have alleged downed Malaysia Airlines Flight 17 in eastern Ukraine last month , killing all 298 people on board .

U.S. officials have concluded the missile was fired from rebel-held territory . Rebel leaders and Russia , which Western nations have accused of supplying the rebels in the months-long fighting , deny involvement .

The ongoing fighting has killed close to 1,400 people -- civilians and combatants -- and more than 4,000 people have been wounded in eastern Ukraine since mid-April , according to U.N. officials . The battles also have forced hundreds of thousands of people to flee their homes and seek shelter either elsewhere in Ukraine or across the border in Russia .

NATO warns Russia

NATO Secretary General Anders Fogh Rasmussen warned Russia Thursday that any further intervention in Ukraine `` would lead to further isolation '' and `` deeper , more profound , tougher economic sanctions that would really hurt the Russian economy . ''

Rasmussen told reporters in Kiev he hopes that Russia , which according to NATO has 20,000 troops along the border with Ukraine , `` steps back because it would n't be in Russia 's interest to intervene further . ''

Russia has denied allegations that it 's supporting separatists in Ukraine and maintains that it wants to see a diplomatic solution to the crisis . But U.S. and Ukrainian officials have accused Russia of saying one thing while doing another -- building up troops along the border and continuing to send support to pro-Russian separatists .

MH17 search efforts halted

The fighting also prompted an international team of experts , led by the Netherlands , to suspend its efforts to search the MH17 debris sites for remaining human remains amid fierce fighting in the area .

The experts said the area was n't safe to search , citing intensified battles in the area . Dutch Prime Minister Mark Rutte told reporters Wednesday that search teams plan to return to the plane crash site . But he did n't give a date for when that might happen .

`` We have done what we could do in the current conditions , '' Rutte said . `` Everyone will agree with us that we should not expose our people to unnecessary risk . ''

Ukraine 's government said Thursday that it was scrapping a cease-fire around the crash site -- put in place to enable the team 's work -- until their mission restarts . In a statement on the Cabinet website , it accused the rebels of provocations and of putting the experts ' lives at risk .

The statement from Ukraine 's Cabinet said at least three more zones where debris lies scattered remain unexamined .

The experts plan to discuss when the next stage of the search operation might begin shortly , it said . Until the decision is made , most will leave Ukraine , but coordination and communication teams will stay .

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CNN 's Nick Paton Walsh reported from Donetsk . CNN 's Jason Hanna and Catherine E. Shoichet wrote and reported from Atlanta . CNN 's Phil Black , Jim Acosta , Isa Soares and journalists Victoria Butenko and Olga Pavlova contributed to this report .

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Rebel leader says his job is done , time to hand over the reins

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Resignation comes as Ukrainian military moves to retake Donetsk

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Ukraine : Militants shot down Ukrainian MiG-29 on Thursday